fix(SingleEntitySelectMenuItems): extract `Add New` button from entitiesToSelect

[nicolasrouanne]

Description

Closes #8169

Extract Add New button from entitiesToSelect and add it as a separate element . There doesn't seem to be a point in having Add New as part of a list, it seems better off in its own component, apart from list items

Rationale

There already is #8353 addressing the same issue, but it seems it doesn't really remove the duplicate "Add New" in the list, leaving a duplicate "Add New" in SingleEntitySelect